Application of PSCF and CF for source identification

The map for Sde Boker in Figure 5 indicates that the Arabian Gulf countries are a regional source for fine vanadium. The map for fine nickel was quite similar to that for fine vanadium. Furthermore, observed fine nickel concentrations were well correlated with fine vanadium concentrations. Both species had average crustal enrichment factors of 24 relative to the average crustal rock composition of Mason (1966), when using aluminium as reference element, suggesting that they have originated from other sources than mineral dust. It is well known that both species are good indicators for residual oil burning (Nriagu and Pacyna, 1988; Chow, 1995). Possible sources of fine vanadium and nickel are at Saudi oil refineries on the Red Sea coast (mainly, Rabigh) and in the Gulf region, or at desalination/power plants. As in the case of arsenic at Sevettijärvi, there are cells with high $P$ values for vanadium inside the Arabian Peninsula that can be attributed to the smearing of the real sources which are closer to the receptor site.

Figure 7 presents the concentration fields obtained by making use of Eq. 3 for various species in the fine size fraction measured at Sevettijärvi. Similar plots have been obtained by employing the PSCF method, and a good correspondence has been found. The major sources of fine lead are located in the Volga region in Russia, in agreement with the location of Pacyna et al. (1995). The same area is the main potential source of fine PM, BC, Zn, and NH$_4^+$. A principal component analysis (PCA) on the concentration data set (De Ridder et al., manuscript in preparation), indicated that these four variables, together with fine S, K, V, and Mn, were all highly loaded on a single component, which was a general pollution component. The CF maps for fine S, K, V, and Mn resembled those for fine PM, BC, Zn, and NH$_4^+$ (the map for fine S was shown in Figure 2), indicating that the various species which were highly loaded on the same pollution component originated from roughly the same source area. That fine PM was highly correlated with the pollution component, and thus also with fine S, is quite logical, as fine sulphate was an important contributor to the fine PM, accounting for nearly 40% on average. It is noteworthy that ammonium correlated well with sulphur, but not with nitrate. According to Seinfeld and Pandis (1998), ammonia is first neutralized by (hydrogen)sulphate and afterwards by nitrate. The average molar ratio of fine ammonium to fine non-sea-salt sulphur at Sevettijärvi was 0.76 (De Ridder et al., manuscript in preparation), thus indicating that sulphate was only partially neutralized to ammonium hydrogensulphate. From the CF maps for fine S and NH$_4^+$ it appears that their source areas and those of their precursor gases (SO$_2$ and NH$_3$) are roughly the same. Fine nitrate was not correlated at all with fine ammonium; it formed a unique component in the PCA (De Ridder et al., manuscript in preparation) and its CF map in Figure 7 indicates that it originates from entirely different source regions than the species that are associated with the general pollution component. The major source regions of fine nitrate are in Western Europe, with maxima at the Benelux countries, Italy and Spain. The maximum in Spain has to be treated with caution, as it is at the edge of the domain. Those at the Benelux countries and in Italy agree with maxima for gridded emission data for NO$_2$ (a precursor of particulate nitrate) (Vestreng, 2001). Furthermore, they agree fairly well with maxima in the European aerosol nitrate concentration field that was derived from quality analysed data by Schaap et al. (2002).

Similar maps as shown in Figures 2 to 7 have been produced for other species and each of the four sites. More specific studies will be presented in forthcoming publications, including an analysis of the effect of changing the air mass arrival height and, in the case of PSCF method, the threshold value. Seasonal variations will also be investigated.